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SubjectRe: [PATCH -mm 08/25] add some sanity checks to get_scan_ratio
On Fri, 06 Jun 2008 16:28:46 -0400
Rik van Riel <riel@redhat.com> wrote:

> From: Rik van Riel <riel@redhat.com>
>
> The access ratio based scan rate determination in get_scan_ratio
> works ok in most situations, but needs to be corrected in some
> corner cases:
> - if we run out of swap space, do not bother scanning the anon LRUs
> - if we have already freed all of the page cache, we need to scan
> the anon LRUs

Strange. We'll never free *all* the pagecache?

> - restore the *actual* access ratio based scan rate algorithm, the
> previous versions of this patch series had the wrong version
> - scale the number of pages added to zone->nr_scan[l]
>
> ...
>
> @@ -1180,15 +1191,19 @@ static void get_scan_ratio(struct zone *
> file = zone_page_state(zone, NR_ACTIVE_FILE) +
> zone_page_state(zone, NR_INACTIVE_FILE);
>
> - rotate_sum = zone->recent_rotated_file + zone->recent_rotated_anon;
> -
> /* Keep a floating average of RECENT references. */
> - if (unlikely(rotate_sum > min(anon, file))) {
> + if (unlikely(zone->recent_scanned_anon > anon / zone->inactive_ratio)) {
> spin_lock_irq(&zone->lru_lock);
> - zone->recent_rotated_file /= 2;
> + zone->recent_scanned_anon /= 2;
> zone->recent_rotated_anon /= 2;
> spin_unlock_irq(&zone->lru_lock);
> - rotate_sum /= 2;
> + }
> +
> + if (unlikely(zone->recent_scanned_file > file / 4)) {

I see nothing in the changelog about this and there are no comments.
How can a reader possibly work out what you were thinking when this
was typed in??

> + spin_lock_irq(&zone->lru_lock);
> + zone->recent_scanned_file /= 2;
> + zone->recent_rotated_file /= 2;
> + spin_unlock_irq(&zone->lru_lock);
> }
>
> /*
> @@ -1201,23 +1216,33 @@ static void get_scan_ratio(struct zone *
> /*
> * anon recent_rotated_anon
> * %anon = 100 * ----------- / ------------------- * IO cost
> - * anon + file rotate_sum
> + * anon + file recent_scanned_anon
> */
> - ap = (anon_prio * anon) / (anon + file + 1);
> - ap *= rotate_sum / (zone->recent_rotated_anon + 1);
> - if (ap == 0)
> - ap = 1;
> - else if (ap > 100)
> - ap = 100;
> - percent[0] = ap;
> -
> - fp = (file_prio * file) / (anon + file + 1);
> - fp *= rotate_sum / (zone->recent_rotated_file + 1);
> - if (fp == 0)
> - fp = 1;
> - else if (fp > 100)
> - fp = 100;
> - percent[1] = fp;
> + ap = (anon_prio + 1) * (zone->recent_scanned_anon + 1);
> + ap /= zone->recent_rotated_anon + 1;
> +
> + fp = (file_prio + 1) * (zone->recent_scanned_file + 1);
> + fp /= zone->recent_rotated_file + 1;
> +
> + /* Normalize to percentages */
> + percent[0] = 100 * ap / (ap + fp + 1);
> + percent[1] = 100 - percent[0];
> +
> + free = zone_page_state(zone, NR_FREE_PAGES);
> +
> + /*
> + * If we have no swap space, do not bother scanning anon pages.
> + */
> + if (nr_swap_pages <= 0) {
> + percent[0] = 0;
> + percent[1] = 100;
> + }
> + /*
> + * If we already freed most file pages, scan the anon pages
> + * regardless of the page access ratios or swappiness setting.
> + */
> + else if (file + free <= zone->pages_high)
> + percent[0] = 100;
> }

Perhaps the (nr_swap_pages <= 0) test could happen earlier on.

Please quadruple-check this code like a paranoid maniac looking for
underflows, overflows and divides-by-zero. Bear in mind that x/(y+1)
can get a div-by-zero for sufficiently-unepected values of y.

The layout of the last bit is misleading, IMO. Better and more typical
would be:

if (nr_swap_pages <= 0) {
/*
* If we have no swap space, do not bother scanning anon pages.
*/
percent[0] = 0;
percent[1] = 100;
} else if (file + free <= zone->pages_high) {
/*
* If we already freed most file pages, scan the anon pages
* regardless of the page access ratios or swappiness setting.
*/
percent[0] = 100;
}

(Was there no need to wite percent[1] here?)

>
> @@ -1238,13 +1263,17 @@ static unsigned long shrink_zone(int pri
> for_each_lru(l) {
> if (scan_global_lru(sc)) {
> int file = is_file_lru(l);
> + int scan;
> /*
> * Add one to nr_to_scan just to make sure that the
> - * kernel will slowly sift through the active list.
> + * kernel will slowly sift through each list.
> */
> - zone->nr_scan[l] += (zone_page_state(zone,
> - NR_INACTIVE_ANON + l) >> priority) + 1;
> - nr[l] = zone->nr_scan[l] * percent[file] / 100;
> + scan = zone_page_state(zone, NR_INACTIVE_ANON + l);
> + scan >>= priority;
> + scan = (scan * percent[file]) / 100;

Oh, so that's what the [0] and [1] in get_scan_ratio() mean. Perhaps
doing this:

if (nr_swap_pages <= 0) {
percent[0] = 0; /* anon */
percent[1] = 100; /* file */

would clarify things. But much better would be

/* comment goes here */
struct scan_ratios {
unsigned long anon;
unsigned long file;
};

no?


> + zone->nr_scan[l] += scan + 1;
> + nr[l] = zone->nr_scan[l];
> if (nr[l] >= sc->swap_cluster_max)
> zone->nr_scan[l] = 0;
> else
> @@ -1261,7 +1290,7 @@ static unsigned long shrink_zone(int pri
> }
>
> while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
> - nr[LRU_INACTIVE_FILE]) {
> + nr[LRU_INACTIVE_FILE]) {
> for_each_lru(l) {
> if (nr[l]) {
> nr_to_scan = min(nr[l],
> @@ -1274,6 +1303,13 @@ static unsigned long shrink_zone(int pri
> }
> }
>
> + /*
> + * Even if we did not try to evict anon pages at all, we want to
> + * rebalance the anon lru active/inactive ratio.
> + */
> + if (scan_global_lru(sc) && inactive_anon_low(zone))
> + shrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0);
> +
> throttle_vm_writeout(sc->gfp_mask);
> return nr_reclaimed;
> }
> Index: linux-2.6.26-rc2-mm1/include/linux/mmzone.h
> ===================================================================
> --- linux-2.6.26-rc2-mm1.orig/include/linux/mmzone.h 2008-05-28 12:09:06.000000000 -0400
> +++ linux-2.6.26-rc2-mm1/include/linux/mmzone.h 2008-05-28 12:11:51.000000000 -0400
> @@ -289,6 +289,8 @@ struct zone {
>
> unsigned long recent_rotated_anon;
> unsigned long recent_rotated_file;
> + unsigned long recent_scanned_anon;
> + unsigned long recent_scanned_file;

I think struct zone is sufficiently important and obscure that
field-by-field /*documentation*/ is needed. Not as kerneldoc, please -
better to do it at the definition site

> unsigned long pages_scanned; /* since last reclaim */
> unsigned long flags; /* zone flags, see below */
> Index: linux-2.6.26-rc2-mm1/mm/page_alloc.c
> ===================================================================
> --- linux-2.6.26-rc2-mm1.orig/mm/page_alloc.c 2008-05-28 12:09:06.000000000 -0400
> +++ linux-2.6.26-rc2-mm1/mm/page_alloc.c 2008-05-28 12:11:51.000000000 -0400
> @@ -3512,7 +3512,8 @@ static void __paginginit free_area_init_
> }
> zone->recent_rotated_anon = 0;
> zone->recent_rotated_file = 0;
> -//TODO recent_scanned_* ???
> + zone->recent_scanned_anon = 0;
> + zone->recent_scanned_file = 0;
> zap_zone_vm_stats(zone);
> zone->flags = 0;
> if (!size)
> Index: linux-2.6.26-rc2-mm1/mm/swap.c
> ===================================================================
> --- linux-2.6.26-rc2-mm1.orig/mm/swap.c 2008-05-28 12:09:06.000000000 -0400
> +++ linux-2.6.26-rc2-mm1/mm/swap.c 2008-05-28 12:11:51.000000000 -0400
> @@ -176,8 +176,8 @@ void activate_page(struct page *page)
>
> spin_lock_irq(&zone->lru_lock);
> if (PageLRU(page) && !PageActive(page)) {
> - int lru = LRU_BASE;
> - lru += page_file_cache(page);
> + int file = page_file_cache(page);
> + int lru = LRU_BASE + file;
> del_page_from_lru_list(zone, page, lru);
>
> SetPageActive(page);
> @@ -185,6 +185,15 @@ void activate_page(struct page *page)
> add_page_to_lru_list(zone, page, lru);
> __count_vm_event(PGACTIVATE);
> mem_cgroup_move_lists(page, true);
> +
> + if (file) {
> + zone->recent_scanned_file++;
> + zone->recent_rotated_file++;
> + } else {
> + /* Can this happen? Maybe through tmpfs... */

What's the status here?

> + zone->recent_scanned_anon++;
> + zone->recent_rotated_anon++;
> + }
> }
> spin_unlock_irq(&zone->lru_lock);
> }



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